A hot electron transistor is a high-frequency, high-speed solid state device, first proposed by Mead in 1960. Efforts were made to develop such a device during the 1960's without success, due to difficulties encountered in its fabrication.
There has recently been an increasing amount of interest in this field because of the development of some new technologies, such as the molecular beam epitaxy. In 1980, Heiblum made a detailed survey of the work carried out in this area and proposed the "heterojunction structure". Using such a structure, preliminary experimental results have been reported by Yokoyama, Hase, and others.
A conventional "heterojunction structure" hot electron transistor is similar in several ways to a bipolar transistor. FIG. 1(a) illustrates the sectional structure, and FIG. 1b) an energy band diagram for such a bipolar transistor. Two electrodes (1) are located on the mesa for connecting to the emitter and the base. The emitter, base and collector layers are formed of GaAs, and 2Ga.sub.0.65 Al.sub.0.35. Barrier layers (2) and (4) are located between these layers.
The transistor comprises two heterojunctions, each consisting of a thin barrier layer and a thick electron drift region. The thick drift regions essentially eliminate variation in barrier shape with bias voltage and reduce junction capacitance.
There are two main problems which occur with the use of this prior art structure:
1. The current transit coefficient, (.alpha.) is small. (.alpha.) is the ratio of the output current to the input current. Owing to the "scattering" in the base and the "quantum mechanics reflection" at the barrier interface, a portion of the injected electrons fall down to the base, so that the value of (.alpha.) is much less than 1, and the gain of the device is low.
2. The emitter-base capacitance is rather large because of very thin barrier layer thickness, so that high frequency performance is limited.